A common type of instrument is one in which a sensor produces a signal whose frequency is related in some functional way, often nonlinear, to a physical input variable. In order to use such a sensor in a digital system, the frequency of the sensor signal must be converted into a series of digital samples. The most versatile systems for measuring frequency operate either by counting cycles of the sensor or input signal for a known time interval, or by counting cycles of a reference signal for an interval controlled by the sensor signal. Using either technique, the resolution of such a frequency measurement depends on the number of counts that can be accumulated during the counting or sampling interval. The resolution may therefore be very poor at high sampling rates.
For wideband sensors such as accelerometers, sampling rates must often be much greater than would otherwise be required by system bandwidth requirements, in order to avoid errors due to frequency aliasing and input rectification. However, if resolution cannot be improved by digital low pass filtration, i.e., by averaging, substantial errors may be introduced by using high sample rates. With conventional frequency counting techniques counts are lost between adjacent sampling intervals, because the counters are reset for each sample. Therefore, with conventional frequency sampling methods, averaging several samples usually does not significantly improve the resolution.